Electrical cables, used for communication or power transmission, often develop faults between insulated conductors, insulated conductors and a metallic shield or armor, insulated conductor and earth, or metallic shield or armor and earth. These faults are usually as a result of damaged or deteriorated insulation which, in the presence of ground water, develop electrical faults. A further complication exists in that watercaused resistive faults are time and voltage variant. A conductor, which is partially shorted due to a water fault, will have a fault resistance which will vary with the applied voltage, the polarity of the applied voltage and with time as electrolysis changes the presence of ions and exposed conductor surface. This variable fault resistance is unpredictable and renders useless simple loop resistance methods.
Several instruments, employing various bridge techniques, are available to measure the electrical resistance of the conductor to the fault which can then be used to calculate the distance to the fault. The bridge measurement methods, particularly when the fault resistance is high relative to the conductor resistance, require a parallel conductor or pair of conductors, to "strap back" the far end of the faulted conductor to the arms of the measurement bridge. If non-faulted conductors are not available, then the bridge measurement methods are not possible.
Another fault location method uses a short electrical pulse which is transmitted down the faulted line. At the fault a portion of the transmitted energy is reflected back to the transmit end where it is detected. The time delay from the launch to return provides a distance estimate to the fault. This method is useful only on certain types of conductors and cannot easily detect high resistance faults.
The present invention relates to the provision of a method whereby the distance to a conductor fault may be determined without the inherent limitation of the previously known methods.